Field of the Invention
This invention relates to structures for resisting seismic loading of structures and, more particularly, to novel systems and methods for retrofitting structures to increase seismic resistance.
Background Art
Seismic retrofitting of “soft, weak, or open front” (SWOF) buildings presents tactical challenges for engineers. Utilities and other obstructions often make moment-resisting frames difficult to install. The most viable retrofit component is often a cantilevered column with an appropriate new foundation.
The SEAOC (Structural Engineers Association of California) Blue Book suggests strategies that rationalize designing cantilevered columns using the R factor for the overall building. Plywood shear walls are typically used as the lateral force resisting system in SWOF buildings. Using the R factor for plywood shear walls instead of R for cantilevered column systems reduces seismic design forces, thus reducing construction costs. However, building officials may not accept the Blue Book rationale; furthermore, designers may wish to increase the ductility of cantilevered columns.
Seismic retrofitting is particularly difficult in established urban areas, such as San Francisco. Many building lots in San Francisco are very narrow, providing limited room for seismic retrofitting structures, requiring cantilevered columns or moment-resisting frames. Likewise, the front of almost all mid-block buildings includes a garage door, main entry door or stairwell, and a door to a service alley. Some buildings have two stairways; some have two garage doors; in some neighborhoods the garage door is recessed inward several feet from the front wall, and the service alley entrance opens from the wall on one side of the recess. Hardly any buildings have more than four feet of unobstructed wall length available for installing shear panels.
Near the front of almost all buildings in San Francisco there are multiple gas and electric meters, either in the garage or service alley. Water service is usually not metered separately, so a single water supply line is typical. In some buildings, the waste plumbing stack from stories passes next to the garage door. Relocation of such structures is extremely expensive. Other obstructions to seismic retrofitting include fire-sprinkler lines, water heater flues and HVAC ducts. Often the garage door hugs the foundation or side wall of the garage and no space is left for vertical members of a moment-resisting frame. Owners of buildings affected by the mandatory soft story retrofit ordinance must often relocate utilities, at substantial added cost, to allow installation of steel frames. For owners who hope to strengthen their buildings voluntarily, the added costs may simply cause them to drop their retrofit desires altogether.
The apparatus and methods disclosed herein provide an improved approach to seismic retrofitting where space is very limited.